The present disclosure relates to a detection method, an inspection method, a detection apparatus, and an inspection apparatus. In particular, the present disclosure relates to a detection method and a detection apparatus for detecting image data that is used for an inspection of an object to be inspected, and an inspection method and an inspection apparatus for inspecting the object to be inspected by using the detected image data for the inspection.
For example, in an inspection for inspecting a mask for EUV (Extreme Ultra Violet) lithography (hereinafter, referred to as a EUV mask), high-intensity pulsed light may be used in order to improve the accuracy of the inspection. Further, critical illumination may be used in order to secure the luminance of illumination light. The critical illumination is a method in which an object is illuminated so that an image of a light source is formed on a top surface of a EUV mask, and uses an optical system capable of illuminating the object with high luminance.
Further, in some cases, when image data for an inspection (hereinafter also referred to as inspection image data) is detected, an XY-direction two-dimensional area sensor is operated in a TDI (Time Delay Integration) mode in which pixel values of the area sensor are transferred in an X-direction while being synchronized with the stage and time delay integration is performed for the obtained pixel values. The use of the TDI mode can compensate for insufficient sensitivity of sensor elements and hence a mask pattern can be photographed with high sensitivity.
A lithography mask inspection apparatus compares a taken inspection image with design data or a reference image that is obtained by photographing (hereinafter also expressed as shooting) the same pattern on a sample. Then, when they do not match each other, the inspection apparatus determines that there is a pattern defect.
When a pulsed light source is used as a light source for an inspection apparatus, in many cases, a luminance distribution of pulsed light emitted from the light source changes from one pulse to another and hence variations occur. Note that the term “variations” in this specification means not only that the total amount of light of pulsed light changes from one pulse to another, but also that a positional distribution of irradiation light intensities on a surface irradiated with pulsed light (hereinafter simply referred to as a luminance distribution) changes from one pulse to another. Such variations are called luminance unevenness.
When there is luminance unevenness in illumination light emitted from a pulsed light source, undesired changes in the luminance (artifacts) occur in a taken mask pattern image according to the luminance unevenness and hence an error occurs in the determination of a pattern defect. Therefore, it is necessary to detect luminance unevenness of the light source and correct output fluctuations of the TDI sensor according to the detected luminance unevenness.
Regarding the change in the total light amount of pulsed light from one pulse to another, as described in Japanese Unexamined Patent Application Publication No. 2010-091552, it is possible to correct output fluctuations of the TDI sensor by installing a correction light amount sensor that detects a light amount of pulsed light and measuring the total light amount of each pulse in synchronization with the cycle of the pulsed light. However, in this document, since the change in the luminance distribution from one pulse to another is not taken into consideration, there is a problem that an artifact caused by this change cannot be corrected.
In order to correct luminance unevenness, it is necessary not only to measure the total light amount of each pulse, but also to use a detector for correction (hereinafter also referred to as a correction detector) capable of detecting a luminance distribution of pulsed lights. Therefore, as a detection apparatus for correcting luminance unevenness, the following method is conceivable. That is, luminance unevenness is corrected by detecting a luminance distribution of a light source using a second TDI sensor.
However, as explained later with reference to examples, in the case in which image data of an object to be inspected and image data of a luminance distribution are acquired by using a TDI sensor, when the number of pixels in a transfer direction in a detector for inspection (hereinafter also referred to as an inspection detector) differs from the number of pixels in a correction detector, there is a possibility that an error is involved due to a difference between the numbers of pulses of light emitted within a cumulative time period of these detectors.
The present disclosure has been made to solve the above-described problem and an object thereof is to provide a detection method, an inspection method, a detection apparatus, and an inspection apparatus capable of preventing an error in a luminance unevenness correction and thereby accurately inspecting an object to be inspected.